There are a few areas on the body that provide important information regarding the state of a horse's circulatory health. These areas include the gums, the pink membrane inside the nostril, the whites of the eyes, and the pink inner vulvar membranes of mares. That said, the simple act of evaluating the color of your horse's gums is a very important skill.

When a horse's gums are pink but have a dark line at the gum-line of the incisor teeth this can be a sign of toxemia, dehydration and other abnormal circulatory states. In the veterinary world, this is often called a "toxic line".

This appearance can also be confused with normal. Keep in mind that horses (especially older horses) with gingivitis, periodontal disease, or EOTRH will also have a red line at the gum margin.